Unit 4 worksheets 5 Intermolecular forces
1. The following five compounds have identical or very similar molar masses. Deduce the increasing
order of their boiling points (lowest first).
ethanol, CH3CH2OH, propane, C3H8, ethanal, CH3CHO, methanoic acid, HCOOH
and methoxymethane, CH3OCH3
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2. Pentane, C5H12, has three structural isomers (compounds with the same molecular formula but a
different structural formula). They are pentane, 2-methylbutane and 2,2-dimethylpropane.
Deduce, with an explanation, the correct order of their increasing boiling points.
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3. Ethanol has a relative molar mass of 46 and boils at 78.5 oC. Water has a relative molar mass of 18 and
boils at 100 oC under the same conditions. Both show hydrogen bonding. Explain why water has a
higher boiling point.
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4. Trichloromethane, CHCl3 has a boiling point of 61.2 oC. Propanone, CH3COCH3, has a boiling point
of 56.2 oC. Explain why a mixture of trichloromethane and propanone can have a higher boiling
point (measured under the same atmospheric conditions) than either of the two pure components.
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5. Place the following four types of attractive forces in order of increasing strength.
Dipole-dipole interactions, hydrogen bonding, London (dispersion) forces and ionic bonding.
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Answers
1. Propane (London dispersion forces, B.Pt. -42.2 oC) < methoxymethane (weak dipole-dipole, B.Pt. 24.8 oC)
< ethanal (dipole-dipole, B.Pt. 20.8 oC) < ethanol (hydrogen bonding, B.Pt. 78.5 oC)
< methanoic acid (strong hydrogen bonding, B.Pt. 101 oC).
2. 2,2-dimethylpropane (B.Pt. 9.5 oC) < 2-methylbutane (B.Pt. 27.9 o C) < pentane (B.Pt. 36.3 oC)
The more spherical the molecule the less surface area there is to attract another molecule of the
same type.
3. Because the oxygen atom in water has two hydrogen atoms bonded to it the polarity of the
molecule is greater than ethanol where only one hydrogen atom is bonded directly to the oxygen
atom. The dipole on the oxygen atom in water will effectively be 2δ– and will attract the
δ+ hydrogen atom of another water molecule more strongly.
4. Both trichloromethane and propanone are polar molecules and there will be dipole-dipole
interactions between the molecules in the pure liquids. When they are mixed the δ+ hydrogen
atom in trichloromethane can form a type of hydrogen bond with the δ– oxygen atom in propanone
resulting in a stronger intermolecular force – more energy will be required to break this
attraction and hence the higher boiling point.
5. London (dispersions) forces < dipole-dipole interactions < hydrogen bonding < ionic bonding